Many mobile devices include both cellular communications functionality (e.g., according to the Long Term Evolution [“LTE”] standard) and location detection functionality (e.g., using Global Positioning System [“GPS”] or Global Navigation Satellite System [“GNSS”] technology). Traditional mobile devices utilize separate and discrete integrated circuits (“ICs”) to implement the cellular communications functionality and the location detection functionality. Each of these separate and discrete IC traditionally has used its own dedicated crystal oscillator (“XO”) for generation of a reference signal that is used for signal processing.
Cellular communications systems and GPS/GNSS systems have differing performance requirements for their respective XOs which typically precludes using a single, shared XO for both of these systems. For example, many GPS/GNSS systems use a temperature-compensated crystal oscillator (“TCXO”) to increase frequency stability over temperature. Whereas, many cellular communications systems have strict phase noise requirements that may be difficult to meet a TCXOs. The competing requirements are discussed further below.
GPS/GNSS systems perform long coherent integrations to increase sensitivity and to improve the signal to noise ratio (“SNR”) of signal measurements. To properly perform these long coherent integrations, a typical GPS signal (e.g., at 1575 MHz) should ideally remain coherent over a typical integration period of a high sensitivity receiver (e.g., fifty milliseconds to a few hundred milliseconds). To maintain high sensitivity, a frequency drift should ideally accumulate to a small fraction of a cycle within these timeframes. Without temperature-compensation, a crystal is generally not able to support the high level of sensitivity required for GPS/GNSS systems, especially in cases where thermal dynamics have a significant impact on sensitivity.
Many cellular communications systems have strict phase noise requirements that require a very high reference frequency that often requires the use of a frequency doubler with the XO. More specifically, there is a limit to the oscillation frequency that can be generated with a fundamental mode crystal, where the frequency limit is approximately 50 MHz. Therefore, a frequency multiplier, such as a doubler to provide an example, is necessarily applied to the output of an “ordinary” XO for use by the cellular communications systems to improve the phase noise. However, the output of a TCXO may be clipped, and/or include small DC offsets that can cause a duty cycle variation in its output, both of which preclude the use of the frequency multiplier in conjunction with the TCXO in cellular communications systems.
Thus, conventional implementations for mobile devices with both cellular communications and location detection functionality use separate crystal oscillators to meet the divergent needs for cellular communications and location detection. This dual-XO implementation strategy has drawbacks, however. For example, using two XOs is more expensive than using a single, shared XO due to the cost of the additional XO and the cost of additional supporting circuitry. Further, the oscillations of any one XO can introduce unwanted interference into circuitry supported by the other XO. Negating the impact of this interference can also require additional circuitry, which can also add to cost and semiconductor real estate. Finally, the use of two XOs complicates the sharing of precise knowledge of time and frequency between the GPS/GNSS and cellular systems.
Features and advantages of the present invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawings, in which like reference characters identify corresponding elements throughout. In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements. The drawing in which an element first appears is indicated by the leftmost digit(s) in the corresponding reference number.